You always add 6 feet to what ever size you build that way you will end up 6 feet short of what you need once you move in.
No matter how big you build it the garage is never big enough.
I did find my first garage 24x 36 with the doors on the long side was not as useful as my 24x 36 with a double door on one short side end and a single on the other short side end. Just more flexibility and shop space
I'm in a 24x36ish currently and with three doors on one of the long walls. It is certainly enough space to be dangerous--especially if I had ceiling height. If only.
"No problem, that should be a one-Saturday job." I've said this myself a whole lot of times, only to have the project stretch into three weeks as things broke, didn't fit, etc.